The present invention relates to a process for early detection of reactor fouling occurring during a gas phase polymerisation. More particularly, the present invention relates to a process for early detection of reactor fouling occurring during a gas phase polymerisation of olefin(s) using a fluidized bed reactor comprising a fluidization grid.
It is known to polymerize one or more monomers in gas phase at a pressure which is higher than atmospheric pressure in a fluidized bed reactor where polymer particles being formed are maintained in the fluidized state by virtue of a reaction gas mixture containing the monomer(s) to be polymerized and travelling in an upward stream. The, polymer thus manufactured in powder form is generally drawn off from the reactor in order to maintain the bed at a more or less constant volume. The process according to the present invention employs a fluidization grid which distributes the reaction gas mixture through the bed. The reaction gas mixture leaving via the top of the fluidized bed reactor is recycled to the base of the latter under the fluidization grid through the intermediacy of an external circulation conduit fitted with a compressor.
In general, the fluidized bed reactors according to the present invention can be represented by a volume whose enclosure (wall) consists of at least one surface of revolution generated by the rotation of a rectilinear and/or curvilinear segment about a vertical axis known as an axis of revolution, and of a disengagement vessel which is above the said surface. The wall of the reactor is therefore a surface of revolution comprising a vertical axis of revolution above which is the enclosure of a disengagement vessel.
Conventional fluidized bed reactors employed for the gas phase polymerization of olefin(s) usually consist of a cylinder (1) with a vertical axis, comprising a fluidization grid at its base, and above which is a disengagement vessel (3), in accordance with FIG. 1, which shows diagrammatically a preferred apparatus for gas phase polymerization according to the present invention. The cylindrical part of the reactor is usually characterised by a height/diameter ratio (H/D) which is comprised between. 1 and 15, preferably between 2 and 8, D representing the internal diameter of the reactor. The disengagement vessel which is above the cylinder capable of containing the fluidized bed has, in principle, a cross-section which is larger than that of the cylinder. It is preferably in the shape of a bulb consisting essentially of a conical frustum of revolution with a vertical axis coinciding with the axis of the cylinder, with an apex pointing downwards with an angle preferably of between 10xc2x0 and 60xc2x0 and having above it a dome of substantially hemispherical shape. The small base of this conical frustum coincides with the upper end of the cylinder of the reactor, and its large base coincides with the base of the dome. It may also consist of a vertical cylinder connected to the cylinder capable of containing the fluidized bed by a connecting surface in the shape of a flared conduit. In this case this cylinder has a vertical axis coinciding with the axis of the cylinder capable of containing the fluidized bed and a roof generally of substantially hemispherical shape.
The essential purpose of the disengagement vessel is to slow down the upward gas stream which, after having passed through the fluidized bed, can entrain relatively large quantities of solid particles. As a result of this, most of the entrained solid particles return directly into the fluidized bed. Only the finest particles can be entrained out of the reactor.
Many malfunctions can occur during a gas phase polymerisation. The major consequence of those malfunctions is the generation of agglomerates in the reactor. It can affect the properties of the produced polymer. It can further affect the fluidization characteristics of the fluidization gas, which can lead to channelling and potential irreversible problems.
This agglomerations formation can occur at any time in the reactor following one or a number of different malfunctions.
For example, a malfunction may create the formation of agglomerates by adhesion of molten particles of catalyst and of polymer to the wall of the reactor, inter alia in the disengagement vessel. Agglomerations formation are usually referred to in the prior art as fouling of the reactor.
When these agglomerates become heavy they can separate from the wall and thus block the fluidization grid and/or the system for withdrawing the polymer.
To prevent the fouling of the reactor from affecting the operation of the polymerization system and the quality of the polymer produced, the reactor is stopped at regular intervals, in order to clean the reactor walls and to extract the agglomerates. This can be done by means of water or nitrogen under pressure. Cleaning of this type brings about the introduction of poisons into the reactor, requiring extensive purges of the reactor and drying in order to remove these poisons. This procedure takes time and is not very economical.
There are a lot of disclosures in prior art of those fouling phenomena as well as many different tentative explanations for their occurrence. Sometimes the type of catalyst used is said to be responsible for the fouling; static electricity has also been indicated as being a cause thereof, operating conditions have also been considered as the most important criteria; in fact, many different theories and proposals for explaining and trying to reduce fouling phenomena have been developed.
It would be a major improvement in the art if a simple method was available for early detection of reactor fouling. An early detection of those problems would then allow an early active step to be taken in order to mitigate or even eliminate said problems. Rapidity of action is indeed crucial in the present technology.
The Applicants have now unexpectedly found a method for detection at the early stage of reactor fouling occurring during gas phase polymerization using a fluidized bed reactor comprising a fluidization grid.
The present invention consequently consists of a process for early detection of reactor fouling occurring during a gas phase polymerisation using a fluidized bed reactor comprising a fluidization grid, characterized in that the upper part of the fluidization grid is fitted with devices capable of detecting the polymer agglomerates failing on and/or hitting said devices.